1. Field of the Invention
The present invention relates to parking brakes for large vehicles, and particularly those actuable through motion of the shift lever for the transmission of the vehicle.
2. Description of the Related Art
In a conventional automatic transmission for an automobile the transmission is locked to prevent movement of the vehicle when the shift lever is in the PARK position. Normally, this is accomplished by having a park pawl engage some portion of the transmision to hold it stationary relative to the transmission housing.
Such a technique cannot easily be used with vehicles having a high gross vehicle weight (GVW). A park pawl normally is not strong enough to hold the vehicle in place. In addition, the higher the loading on a park pawl, the more difficult it is to disengage. Instead, the parking brake for the vehicle must be activated.
Conventionally, the parking brake is activated through some control mechanism, e.g., a pedal, lever or switch, entirely separate from the transmission shift lever. This generally does not pose a problem for professional drivers used to driving large vehicles, since they activate the parking brake as a matter of habit any time they are leaving the vehicle. Similarly, anyone who normally drives a vehicle with a manual transmission normally engages the parking brake whenever they leave the vehicle.
Real problems arise with the occasional driver of a large GVW vehicle who normally drives an automobile with an automatic transmission. Unless they live in a particularly hilly region, such people do not engage the parking brake as a matter of course. They simply rely on the park pawl in the transmission.
It therefore is desirable to employ some mechanism to automatically engage the parking brakes via movement of the transmission shift lever in a vehicle which may be driven by people who only infrequently drive high GVW vehicles. Typical examples of where such a mechanism would be desired include large recreational vehicles, such as motor homes, and large trucks for use by rental companies. Of course, it would also be more convenient even for professional drivers if their parking brake were engaged and disengaged automatically.
One such system for applying the parking brake via movement of the transmission shift lever is shown in FIG. 1. This system employs a spring-activated, hydraulically de-activated parking brake a controlled by a cable-operated valve b. The valve b is opened to reduce hydraulic pressure and allow the brake a to engage via a cable c connected to the transmission shift lever d. However, with this cable controlled valve b, a separate Bowden-type cable e is necessary to de-activate the parking brake a, and this necessitates a separate brake release control f on the vehicle. This has the disadvantage that the brake a must be released by a motion separate from moving the shift lever out of the PARK position. If the operator neglects to release the brake in this fashion, it may burn out.